Tendon injuries constitute an increasing proportion of trauma cases reported at our hospital, yet research in the molecular biology of tendon healing has been neglected. The major responses of cells in an injured tendon is to produce new collagen and noncollagen proteins. Collagen is the key structural protein that persists, giving tendon its ability to resist tensile forces. The collagen crosslink content and location largely determine the magnitude of the resistance to tensile force. It is known that immobilization of an injured limb leads to joint stiffness and intractable adhesions in ligaments and tendons that impair mobility. An increase in metastable, reducible collagen crosslinks accompanies loss of mobility. Mild exercise of an injured tendon as early as possible after surgical repair is favorable to healing because certain metastable, reducible, and stable pyridinoline crosslinks may not form to limit motion. How do tendon cells react to and regulate the collagen response to wounding, and how does immobilization or motion alter these responses? The overall objectives of this study are to identify changes in the structure of collagen in tendons that are healing and have been subjected to immobilization or motion. Based on results of animal experimentation, we hope to apply a strategy to human medicine involving physical and chemical intervention to improve tendon healing.